One way to determine if the plugs have the correct heat range for the engine build is to look at where the color change occurs on the spark plug ground strap. The color change should occur about 1/2 way along the length of the strap, so about in the middle of the 90 degree bend.
If the color change is not easily located on your existing plugs, you might need to install fresh new plugs, then run it a couple of standing start full throttle passes, and check where the color change is located.
